The present invention pertains to the construction of an electromagnetic device, such as an electric motor, having a segmented stator with stator teeth held in a circular pattern solely by a shell of the motor that has been hot dropped over the stator teeth.
Current trends in the design of electromagnetic machines such as motors have led to compact designs of high efficiency motors. The motor designs have obtained high output power to volume ratios through their use of high magnetic flux density (or xe2x80x9chigh remanencexe2x80x9d) magnets on their rotors and high density windings of their stators, increasing efficiency, and through optimized thermal design which increases the motor""s ability to dissipate losses.
With the reduction in size of these high efficiency motors the precision with which their components"" parts are assembled becomes more important. Specifically, as the size of a motor becomes increasingly smaller, the size and accuracy with which the air gap (which separates the exterior surface of its rotor from the interior surface of its stator) must be similarly reduced in order to compare favorably to a larger model with similar performance characteristics. In addition, with decreasing motor size, the tolerances of the bearings and their associated mounting diameters, rotor shaft and stator bore center axes also decrease, and the slightest misalignment can result in negative effects on motor performance, in increased bearing wear which significantly decreases the operational life of the motor or in contact of the rotating rotor with the stator bore which prevents its proper functioning all together.
Compact high efficiency motors are constructed of basically the same component parts typical to most motors, those being the stator, which is the stationary electromagnetic component of the motor, the rotor, which is the rotating electromagnetic component of the motor, and the endbells, which locate the rotor in relationship to the stator. To achieve the necessary tolerances for the motor""s compact size, each of the component parts of the motor must be machined and assembled with high accuracy relative to the other component parts of the motor. The stator must be assembled in the motor housing shell and the motor housing shell must be manufactured to align or register the center axes of the stator bore relative to the housing shell. The endbells are machined with reference to the stator center bore. By machining and assembling each of the component parts of the motor with reference to the other component parts of the motor, the center axis of the rotor is closely registered with the center axes of the bearings mounted in the endbells which, in turn, are registered with the center axis of the stator bore when the component parts are assembled in the motor. The precise machining and assembly of the motor component parts is necessary to properly position the rotor in the stator bore and the rotor bearings in the endbell bearing seats. The extremely precise machining and assembly of the motor component parts comprise a major portion of the expense involved in manufacturing compact, high efficiency motors.
The present invention is an electromagnetic device, such as a motor, having component parts and a method of assembly that provide a motor of compact size which provides higher output performance and higher efficiency than similarly sized motors. The novel features of the invention are in the constructions of its component parts and in their method of assembly and, although described as applied to a motor, they may also be applied to alternators and generators. The improvements accomplished by these specific design and manufacturing techniques give higher torque density and improved thermal conductivity (allowing the motor to dissipate any losses more effectively). The design concepts under consideration here result in a device which is optimized to minimize cogging and torque ripple and provide uniform back EMF, which are significant contributors to output motion quality.
The motor of the invention is basically comprised of a stator assembly consisting of a wound stator core contained in a housing shell with a pair of endbells attached to the opposite ends of the housing shell, impregnating resin or encapsulant, and a rotor assembly. The novel features of the motor are in its component parts and the method in which they enclose the electromagnetic device, i.e., the stator and rotor of the motor. Therefore, the stator construction and rotor construction are described in only general terms with it being understood that alternative stator and rotor constructions may be employed with the invention.
The stator is a segmented stator comprised of stacks of stator laminates with each stack surrounded by an individual winding. Wound stacks are arranged in a circle in preparation for their being assembled with the housing shell.
The housing shell is tubular having a hollow interior and openings to the interior in opposite first and second end surfaces of the shell. The interior of the shell is machined to a precise diameter, and then the opposite first and second end surfaces are machined flat and perpendicular to the center axis of the shell interior. A series of pin holes is machined into each of the end surfaces of a specific depth to be described below. The shell is heated, allowing it to expand slightly, and then is hot-dropped over the circular cluster of wound stator stacks in precise alignment to the orientation of the stacks. A printed circuit board is then connected to the terminals of the stator windings and is positioned so that it is adjacent the stator windings at the rear of the stator assembly.
Both the front and rear or first and second endbells are cast from aluminum (although other materials may also be used). Steel bearing support rings are centered in the endbells as they are cast with a larger of the two bearing support rings being cast into the forward or first endbell. The endbells then receive basic machining creating a series of fastener through holes and threaded holes, and creating mating surfaces on the endbells having pilot holes machined therein. Steel pins are inserted into the pilot holes.
The endbells are positioned so that they are adjacent the opposite first and second end surfaces of the housing shell with the front end bell positioned adjacent to the first end surface and the rear end bell positioned adjacent to the second end surface. The endbell center axises are aligned with the axis of the stator bore, with the steel shear pins closely related with the matching pilot holes drilled into the housing shell. The end bells are then pressed into position over the first and second end surfaces of the housing shell with the end bell pins broaching into the pin holes of the shell end surfaces providing a precise and tolerance independent fit of the end bells over the opposite first and second end surfaces of the housing shell. The depth of the receiving holes in the housing shell is such there will be sufficient space at the bottom of the drilled hole to receive the shavings produced by the broaching process. The pins resist relative shear and torsional forces between the endbells and the housing shell. Bolts are inserted through the through holes in the front endbell and are screwed into a fastener threaded holes in the rear endbell in order to further secure in tension the endbells on the opposite end surfaces of the housing shell.
A removable core fixture assembly is inserted through a shaft opening of one of the endbell bearing support rings, through the stator bore, and through the shaft opening in the opposite endbell bearing support ring in preparation for injection of the encapsulant. An impregnating resin or encapsulant is then injected through one or more of the series of injection openings in one of the endbells. The encapsulant flows axially through the stator assembly permeating the stator core and the endbells until it passes through the injection venting openings of the opposite endbell. The core assembly excludes this material from the bore and bearing regions of the stator assembly. The encapsulant is cured and the core and associated fixturing are removed.
The front or first end bell""s bearing bore is then machined in the steel bearing support ring cast into the front endbell. The front bearing bore is machined with its center axis referenced from or coaxially aligned with the center axis of the stator bore and axially referenced from the front of the stator wound core assembly. Either simultaneously or in a subsequent operation, the rear bearing bore is machined in the steel bearing support ring cast into the rear endbell. The rear bearing bore diameter is referenced from the stator bore diameter.
Front and rear retainer features are then machined into the front and rear end bells, machined concentrically to and referenced from the stator bore center axis. The axial locations of these features are referenced from or are in register with the axial depth of the front bearing bore.
The rotor is comprised of a one piece magnetic steel rotor shaft and core combination having a series of magnetic rings bonded on its exterior. Ball bearings are pressed to precise locations on the opposite ends of the rotor shaft at opposite ends of the magnet rings with the bearing on the rear end of the shaft having a smaller diameter than either the bearing on the front end of the shaft or the magnet ring, and with the bearing on the front end of the shaft having a larger diameter than that of the rotor core which, for example, may be comprised of the outer diametral surface of the magnet ring.
The rotor is held in precise alignment with the stator assembly and inserted into the stator by first inserting the rearward end of the rotor with its smaller bearing through the larger bearing bore at the front endbell of the rotor. The rearward end of the shaft and its smaller bearing pass through the stator bore until the rear bearing is positioned adjacently to the bearing bore in the rear endbell and the front bearing is positioned adjacently to the bearing bore in the front endbell. The rotor is then pressed into place with a press fit of the outer race of the front bearing in its housing and with a transitional or close slip fit of the rear bearing in its housing. The front bearing is pressed into the bearing support ring in the front endbell until it engages against the annular shoulder formed in the bearing support ring. A front bearing retainer device is then installed at the front of the larger bearing to help prevent long-term creepage. A bearing preload spring is then placed over the rear end of the rotor shaft and against the outer race of the rear bearing. The rear bearing retainer is then placed over the rear end of the shaft and against the preload spring and is secured to the rear endbell. The rear retainer is positioned in an annular seat that has been precisely machined in axial relation to the front bearing seat, resulting in the virtual elimination of variation in bearing preloading due to tolerance stack up. The rear bearing retainer bore is machined to precise concentricity with the stator bore in order to allow the accurate location of feedback devices relative to the rotor and stator assemblies.
The construction of the motor and its method of assembly maintains precision positioning of the rotor at the center of the stator bore with a uniform air gap between the stator bore interior surface and the rotor exterior surface and with the rotor center axis precisely aligned with the center axis of the stator bore as well as the center axes of the rotor bearings.